Field of the Invention
The invention concerns a process for determining the oxygen saturation rate of the arterial blood of a subject, by comparison of the concentration of oxyhemoglobin, or HbO.sub.2, with the total hemoglobin concentration, in which an opto-electronic sensor is used to acquire representative signals of optical absorption on two paths, crossing an appropriate tissue region of the subject, and emanating from two respective light sources, each emitting in a different wavelength band, one in the near infrared and the second in the deep red, so that these representative signals comprise a variable component corresponding to the absorption by the pulsatile arterial blood, and a continuous component corresponding to the absorption by the other tissues crossed, the variable component of each representative signal being separated, and, using known formulas, the respective rates of HbO.sub.2 and deoxyhemoglobin or Hb are calculated from the variable components, and, also using a known method, the oxygen saturation rate is determined therefrom.
The invention also concerns a device for carrying out the method of the invention.
The determination of oxygen saturation rate of the arterial blood helps to determine the capacity of the organism of a subject to respond to the oxygen requirements of the different organs, and, consequently, to appreciate the limits of physical activity that are tolerable without danger, or the need for oxygen therapy.
In a conventional manner, the oxygen saturation rate of the arterial blood, or SaO.sub.2, is determined by sampling the arterial blood, and analyzing this blood. These determinations are made at a given time, and do not provide information about variations in time in response to organic requirements.
Between the years 1930 and 1950, oxymeters were designed in particular to monitor the severe hypoxias to which pilots were subjected (Nicolai 1932, Matthes 1935, Squire 1940, Millikan 1942, Wood and Geraci 1949), the procedure involving a measurement of the optical absorption by the tissues, in order to determine a total absorption of the tissues which are drained of blood by compression using a balloon.
Aoyagi (1974) developed a method for the continuous measurement of the arterial saturation, called pulse oxymetry. This method is based on the following considerations.
The soft tissues are relatively transparent in the deep red and the near infrared. PA1 By contrast, hemoglobin, in its different forms including oxyhemoglobin, and deoxyhemoglobin, exhibit pronounced absorptions in these parts of the spectrum. PA1 Oxyhemoglobin displays an absorption curve as a function of wavelength which, in comparison with the absorption curve of deoxyhemoglobin, is lower in the deep red, and higher in the near infrared, so that, by using two measurements at different wavelengths, one in the near infrared and the second in the deep red, knowing the specific absorption coefficients of the two forms of hemoglobin, one can accordingly calculate the relative contents of these two forms, in so far as the only absorptions involved are produced by the forms of hemoglobin. PA1 In the arterial network, the blood flow is pulled to the heart beat. In a tissue region crossed by the optical path between a light source and an opto-electronic sensor, the volume of arterial blood varies with the pulse rate, and hence the absorption of the arterial blood. By contrast, absorption by the tissues in the above tissue region, other than the arterial blood, does not vary, whether for the immobile soft tissues, or the venous blood at constant flow. In consequence, the signal from the opto-electronic sensor comprises a continuous component, corresponding to the absorption by the tissues other than the arterial blood, and a variable component at the rate of the blood pulse rate, which depends only on the blood absorption. PA1 Strictly speaking, another component of the blood exists, which exhibits pronounced absorptions in the region ranging from the near infrared to the deep red, namely cytochrome C oxidase. However, this component also displays a variable absorption with the oxygen concentration in the blood, and in low concentrations compared with the hemoglobin content, so that, if corrections are not introduced to take account of the specific absorption of cytochrome C oxidase, errors in the oxygen saturation remain of the second order.
The Aoyagi method was improved by Nakajima et al (1975), by Yoshiya et al (1980), and Shimada et al (1984), in particular thanks to advances in opto-electronics, and in signal processing electronics.
J. P. Payne and J. W. Severinghaus (1985) published investigations on the absorption curves of the forms of hemoglobin, which recommend the use of two wavelengths, namely 940 nm in the near infrared, and 660 nm in the deep red.
To calculate the oxygen saturation, which is denoted SaO.sub.2, the following considerations are applied.
The absorption, expressed by the optical density DO, is governed by the Beer-Lambert Law: EQU DO=e.multidot.c.multidot.l (1)
where e is the molecular extinction coefficient, c is the concentration of the absorbent component, and l is the length of the optical path in the absorbent medium.
In fact, the total optical density is the sum of the optical densities due to the specific absorptions of the different absorbents.
By proceeding at two different wavelengths, a first degree system of equations with two unknowns is obtained, the unknowns being the oxyhemoglobin and deoxyhemoglobin concentrations, or HbO.sub.2 and Hb respectively: ##EQU1## where e.sub.1 and e.sub.3 are the molecular absorption coefficients of deoxyhemoglobin, and e.sub.2 and e.sub.4 those of oxyhemoglobin, at 660 and 940 nm respectively.
The resolution of the system (2) is standard, and gives Hb and HbO.sub.2, to within a factor of l. Obviously, the paths in the tissues are equal for both of the wavelengths, which presume that these paths are very near to each other.
The oxygen saturation SaO.sub.2 is accordingly given by: EQU SaO.sub.2 =HbO.sub.2 /(Hb+HbO.sub.2) (3)
where the factor l disappears.
It should be clear that the values of DO considered above correspond to the variable component of the signal indicating absorption, owing to the linear character of the combination of the optical densities.
So far, pulse oxymeters were relatively heavy and bulky instruments, only employed in hospital facilities, where it was impossible to take measurements of the oxygen saturation of the arterial blood continuously in conditions similar to those imposed by everyday life. Thus invaluable information could not be obtained on the risks of hypoxia from the standpoints of diagnosis, treatment and lifestyle.
Furthermore, the oxymetries only gave measurements of Hb and HbO.sub.2, ignoring a third form of hemoglobin, carboxyhemoglobin, which results from the fixation of carbon monoxide on hemoglobin, with a relatively high bonding energy.
For a long time, certainly, apart from cases of serious carbon monoxide poisoning, the error introduced by the presence of carboxyhemoglobin, or HbCO, in the measurement of oxygen saturation, was ignored. With the spreading battle against smoking and air pollution by automotive vehicles, however, the possibility of determining the HbCO content emerged as increasingly desirable.
The objective of the Applicants was to make an instrument capable of determining the respective shares of Hb, HbO.sub.2 and HbCO in the arterial blood, which is readily portable, to be able to determine these Hb, HbO.sub.2 and HbCO concentrations on a subject, over a prolonged period, for example a period of 24 h, with the subject exerting activity close to normal.